Chapter 12 - Forces

1 - Newton’s First and Second Laws

2 - Gravity

3 - Newton’s Third Law

TN Standards

• CLE.3202.3.2: Investigate and apply Newton’s three laws of motion

• CLE.3202.4.1: Explore the difference between mass and weight

• CLE.3202.4.2: Relate gravitational force to mass• CLE.3202.3.3: Examine the Law of Conservation of

Momentum in real-world situations• CLE.3202.Math.1: Understand the mathematical

principles behind the science of Physics

Bellwork

• How many laws of motion did Newton create?

1 – Newton’s 1st and 2nd Laws

• Key Questions:

• What makes an object speed up, slow down, or change directions?

• What determines how much an object speeds up or slows down?

Newton’s First Law• An object at rest remains at rest and an object in

motion maintains its velocity unless it experiences a net force

• Objects change their state of motion only when a net force is applied

• Objects tend to maintain their state of motion

Inertia• “Laziness” of all matter based on mass

• Is the tendency of an object to stay at rest or to move at constant velocity ( if moving )

• Seat belts and car seats

QuickLab – Newton’s 1st Law

• Place an Index Card over a glass; Coin on card

• Flick the card sideways off the glass – what happens to the coin?

• Does coin move with Index Card?

• Repeat but slowly pull card sideways – what happens to the coin?

• Explain the results using Newton’s 1st Law – In your notebook ( lab grade )

Newton’s Second Law

• The unbalanced force acting on an object equals the object’s mass times its acceleration

• Net force equals mass times acceleration– F = ma

• Unbalanced force on an object determines how much an object speeds up or slows down

Newton’s Second Law

• Force is measured in Newtons ( 1 N = 1 kg x 1 m/s2 )• For equal forces, a larger mass accelerates less• Acceleration depends on force and mass

Newton’s Second Law

• Force is measured in Newtons ( 1 N = 1 kg x 1 m/s2 )• For equal mass, a larger force accelerates more• Acceleration depends on force and mass

1 – Newton’s 1st and 2nd Laws

• Key Questions:

• What makes an object speed up, slow down, or change directions?

• What determines how much an object speeds up or slows down?

Bellwork

• What is Newton’s First Law (in your own words)• What is Newton’s Second Law (in your own words)

2 – Gravity• Key Questions:

• How are weight and mass related?• Why do objects fall to the ground when

dropped?• What is the relationship between free-fall

acceleration and mass?• Why does a projectile follow a curved path?

Weight and Mass• Force on an object due to

gravity is WEIGHT– Weight is measured in Newtons

• Free-fall acceleration is a constant acceleration all objects on earth experience

• Weight is equal to mass times free-fall acceleration– w = mg

Law of Universal Gravitation

Law of Universal Gravitation

• All matter is affected by gravity• Gravitational force increases with mass OR

decreases with distance

Free Fall• Without air resistance, all objects

falling near Earth’s surface accelerate at the same rate regardless of their mass

• Due to the Law of Gravitation

• Air resistance can balance weight– EXAMPLE : Leaf falling at constant

velocity– TERMINAL VELOCITY ( max velocity

obtained )

Projectile Motion

• Curved path of an object thrown, launched, kicked, or otherwise projected near Earth’s surface

Projectile Motion• Two components – horizontal and vertical– Curved path when both are combined

Projectile Motion

• Orbiting is projectile motion

2 – Gravity• Key Questions:

• How are weight and mass related?• Why do objects fall to the ground when

dropped?• What is the relationship between free-fall

acceleration and mass?• Why does a projectile follow a curved path?

Bellwork

• What is the difference between weight and mass?

3 – Newton’s Third Law

• Key Questions:

• What happens when an object exerts a force on another object?

• How do you calculate the momentum of an object?

• What is the total momentum after objects collide?

Action and Reaction Forces

• Foot on Ball – ACTION

• Ball on Foot – REACTION

Newton’s Third Law

• For every ACTION force there is an opposite and equal REACTION force

Newton’s Third Law

• Forces always occur in pairs

• Forces in a force pair do not act on the same object– Do not cancel each other

• Equal forces do not always have equal effects

QuickLab – Action/Reaction Forces

• Hang a 2 kg mass from a spring scale

• Observe and record the reading on the spring scale

• With the mass on the first scale, link a second scale to the first

• Observe/record readings from both scales

QuickLab – Action/Reaction Forces

• QUESTIONS:

• What are the action and reaction forces in the spring scale-mass system that you have constructed?

• How did the readings on both scales compare in the last step? Explain how this demonstrates Newton’s 3rd Law?

Momentum

• A property of all moving objects

• Along a straight line, momentum is the product of an object’s mass and velocity– p = mv

• Increases as mass and velocity increases

• Force is related to change in momentum

• How does moving the catcher’s glove back change the force needed to stop the ball?

Conservation of Momentum

• The total amount of momentum in an isolated system is conserved

• Total momentum of two or more objects after a collision is the same as the momentum before

• Explained by 3rd Law

3 – Newton’s Third Law

• Key Questions:

• What happens when an object exerts a force on another object?

• How do you calculate the momentum of an object?

• What is the total momentum after objects collide?

Bellwork

• What is Newton’s 3rd Law ( in your own words )?